The conventional miniaturization of metal oxide semiconductor field effect transistors (MOSFETs) has been the driving force in the technology advancement for the semiconductor industry for more than three decades. However, as the device scale reaches the nanometer regime and below, it is increasingly difficult to sustrain the advancement by conventional MOSFET miniaturization. Therefore, methods for improving performance, without scaling, have become increasingly critical. One general approach for doing this is to increase carrier (electron and/or hole) mobilities in MOSFETs. Use of stress on MOSFETs has proved to be an effective way to enhance carrier mobilities in the transistor channel.
As shown in FIG. 1, different semiconductor devices need different strain configuration for optimal performance. For example, silicon-based p-channel field effect transistors (pFETs) require tensile strain in the transverse direction and compressive strain in the longitudinal direction. In contrast, silicon-based n-channel field effect transistors (nFETs) require tensile strain in both transverse and compressive directions.
Biaxial-strained semiconductor substrates have drawn attention for their use in enhancing device performance. For example, epitaxial silicon-germanium (SiGe) grown on silicon substrates is under biaxial compression since the lattice constant of SiGe is larger than that of silicon substrates. Biaxial-strained SiGe is used for high performance pFETs. However, the biaxial compressive strain is not an optimal configuration since the compressive strain in the transverse direction would degrade pFET performance.
In the prior art, it has been demonstrated that uniaxial compressive strain in the longitudinal direction can be achieved by patterning a biaxial compressive SiGe film into a narrow width structure (on the order of about 500 nm or less) as is shown in FIGS. 2A-2B.
Specifically, FIG. 2A illustrates an initial structure 10 that is used in the prior art in which reference numeral 12 represents a substrate that can be a semiconductor material such as silicon, or a buried insulating material such as a buried oxide (BOX). The initial structure 10 of the prior art shown in FIG. 2A also includes a continuous strained film 14 on top of the substrate 12. The continuous strained film 14, which is formed by conventional techniques such as epitaxial growth, is typically SiGe or any other semiconductor whose lattice constant is larger than that of the substrate 12 for the case in which the substrate 12 is a semiconductor such as silicon. If the substrate 12 is a buried oxide, the continuous strained film 14 may be fabricated by wafer bonding.
Photolithography and etching are used to pattern the continuous strained film 14 into a patterned strained film 14′. The strain at each of the edges 16 of the patterned film 14′ is relaxed because each edge is now a free surface. This edge relaxation extends from each edge 16 inward towards the middle of the patterned structure. The arrows in FIG. 2B show the direction of the inward edge relaxation for the case in which continuous strained film 14 is tensile. When the width of the patterned structure (i.e., patterned strained film 14′) is comparable with its thickness, the edge relaxation will reach the entire pattern and cause the uniaxial strain relaxation laterally.
It is clear from FIGS. 2A-2B that this prior art approach relies on the patterning of the strained film, which would impose requirements on the device geometry and introduce complexity to device design.
In view of the above, it would be desirable to have a method of achieving uniaxial strain relaxation without film patterning.